4,4,4, | Dr. A. Holmes and Dr. S. Smiath— 
ends. Continuation beyond the limits shown on the map cannot be 
proved, and that it persists at the surface is unlikely. At the 
terminations the intrusion is responsible for marked surface features, 
but throughout the rest of its course it merely constitutes the back- 
bone of a ridge which is but one of a series with a common trend. 
The western feature forms the crest of the divide between the rivers 
before mentioned—the Tees and the Gaunless—but the eastern one 
lies within the drainage of the latter stream. 
THE CLEVELAND AND Herr Dykgs. 
Before proceeding to discuss the petrologica] characters of the 
intrusion and the wider problems which then arise, some brief notice 
must be taken of the other two dykes which traverse the region, 
namely, the Cleveland and Hett Dykes. 
It will be seen from the map of the area (Fig. 3) that the 
Wackerfield Dyke, if it persisted only another 500 yards, would 
cross the path of the Cleveland Dyke, but the latter appears to be 
absent at the surface between the Bolam and Buck Head laccoliths. 
There are no surface indications of the Cleveland Dyke between the 
laccoliths and, moreover, colliery workings have crossed the line of 
the dyke without encountering the intrusion. 
It would be out of place to discuss here, except in bare outline, the 
habits of this great dyke and the character of its apophyses,. 
particularly as we hope at some future date to deal with the intrusion 
in detail, but the following facts may be stated. The dyke runs in 
an H.S.E.-W.N.W. direction from the vicinity of Whitby to the 
neighbourhood of Carlisle, but it is not persistently present at the 
surface throughout its long course. East of the Bolam sector, 
which terminates a mile and a half east of our map, it has not been 
proved within 14 miles; where next seen it is traversing Triassic 
beds at Coatham Stob, 34 miles 8.W. of Stockton. Within the 
neighbourhood with which the present paper is concerned it spreads 
out into a number of sills and laccoliths; the laccolith north of 
Bolam village is by far the largest of these. Igneous rock has been 
injected between the strata on both sides of the dyke along a length 
of some 1,200 yards. Although the laccolith attains its greatest 
thickness on the south side of the dyke, its lateral extension is greatest 
on the north, ‘The igneous sheet is concealed to a very large extent 
by sandstone and the boundary we have mapped can be merely 
conjectural. The igneous rock, overlain by sandstone, has been 
exposed in a small quarry marked “a’”’, nearly 400 yards away from 
the dyke, and further east a little distance beyond the limits of our 
map there are traces of whinstone workings 1,200 yards north of 
the dyke. The stone wrought in the old quarry in question may have 
been part of the laccolith, or may indicate a branch from the main 
dyke, the interpretation of the Geological Survey. We have as yet 
insufficient data to justify any decision. 
At Buck Head the injection is wholly on the south side of the 
